Passenger tires often contain a rubber tread of a cap/base construction. The tread cap provides the running surface of the tread and the underlying tread base layer presents a transition zone between the tread cap and tire carcass.
Historically, the tread cap may be a silica-rich rubber composition and the tread base layer may be a carbon black-rich rubber composition.
Conventionally, for a passenger tire, the tread base is typically a relatively thin rubber layer and the tread cap is typically a relatively thick, major portion of such cap/base construction.
However, silica reinforcement is of a significantly greater cost than carbon black reinforcement thereby providing an incentive for creating a novel tire tread construction of reduced thickness for a silica-rich tread cap layer where silica is the major reinforcing filler.
Therefore, it is desired to provide a tread having a silica-rich tread cap layer of reduced thickness relative to the underlying tread base, particularly for use in a tire for a passenger vehicle.
Accordingly, for this invention, a tread is provided as a tread composite comprised of a relatively thin silica-rich tread cap layer and an underlying, integral, relatively thick transition zone composite. The transition zone composite is comprised of an intermediate carbon black-rich layer underlying said tread cap layer and a carbon black-rich tread base layer underlying the intermediate layer. In such construction, the intermediate layer is thereby positioned between said tread cap layer and tread base layer to enable a transition zone of graduated physical properties and, in turn, enable use of a thinner silica-rich tread cap layer.
For such purpose, the tread cap layer in combination with the intermediate layer and base layer of the transition zone composite are required to have a graduated stiffness property and allocated rubber compositions of glass transition temperature characteristics together with allocated silica and carbon black reinforcement for the various layers of the tread composite in which the transition zone composite plays an integral and significant part.
For such purpose, by adjusting the composition, physical property and thickness aspects of the layered tread composite, a carbon black-rich transition zone composite comprised of a cooperative combination of a base layer and intermediate layer in a manner that enables a reduced thickness of the silica-rich tread cap without significantly compromising the effectiveness of the tread cap running surface of the overall tread composite.
As a result, a tire tread is provided in which the transition zone composite is a significant portion of the thickness of the tread composite.
In the description of this invention, the terms “rubber” and “elastomer” where used herein, are used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, where used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients” and the term “compound” relates to a “rubber composition” unless otherwise indicated. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated. The term “Tg” means the glass transition temperature of an elastomer determined by DSC (differential scanning calorimeter) at a heating rate of 10° C. per minute.